1. Field of the Invention
The present invention relates to a communication apparatus, a communication system, a method of processing by the communication apparatus and a non-transitory computer-readable storage medium.
2. Description of the Related Art
An infrastructure mode network, which is composed of multiple stations and an access point, is known as one form of wireless LAN (Local-Area Network) based upon the IEEE 802.11 standards. Devices having a wireless LAN function include a dual mode device possessing the functions of both an access point and a station.
A dual mode device can itself serve as an access point and construct a network, and can participate as a station (client) in a network constructed by another access point. With ordinary CSMA/CA-based access control, each device is in transmission standby if the communication channel is in use and commences data transmission when the communication channel becomes available.
When a plurality of devices waiting for channel availability start transmitting all at once, however, the probability that packets will collide in space rises. In order to avoid this, use is made of a scheme referred to as a “back-off algorithm”.
This scheme is such that after the communication channel becomes available, each device marks time in accordance with the number of random numbers generated by each of the devices. Back-off time is decided by multiplying the value of the random number by a fixed length of time (slot time).
Specifically, back-off time is found by the following equation:back-off time=random-number value×slot time
Random-number values are random integers generated over a range from zero to a contention window (CW). The value of CW is variable and is decided within a range delimited by a minimum value CWmin and a maximum value CWmax.
Specifically, the range is as follows:CWmin≦CW≦CWmax 
The IEEE 802.11e standards introduce the concept of QoS (Quality of Service) and standardizes the technique for assuring data-transmission priority and band depending upon the data content and usage thereof. At present the Wi-Fi Alliance is standardizing IEEE 802.11e-compliant specifications referred to as WMM (Wi-Fi Multimedia) for the purpose of transmitting video and voice streaming data efficiently over a wireless LAN.
WMM identifies transmitting and receiving data using access categories (AC) which define four priorities, namely AC_VO (voice), AC_VI (video), AC_BE (ordinary data) and AC_BK (mail and other data).
FIG. 9 is a diagram explaining priority processing classified by access category of WMM, which is an access control scheme representing an expansion of the CSMA/CA scheme according to the prior art.
A WMM-compatible access point possesses a management table relating to QoS parameters and, together with a WMM-compatible communication device, exercises access control on a per-access-category basis.
AIFS (Arbitration Inter-Frame Space) is a frame transmission interval and refers to minimum standby time, which extends from the moment a communication channel is determined to be idle to the moment data transmission starts. This means that the smaller the value of the AIFS, the shorter the time is takes for data to be capable of being transmitted and, hence, the higher the transmission priority of this data.
Thus, with WMM, QoS is realized over wireless media by using the ACs that define the four priorities and the multiple QoS parameters (AIFS, CWmin, CWmax) for access control.
By way of example, the specification of Japanese Patent Application Laid-Open No. 2005-012725 discloses a CSMA-based communication system in which the QoS parameters (AIFS, CWmin, CWmax) are changed dynamically and priority of communication data controlled in accordance with communication status and the intent of communication management.
Further, the specification of Japanese Patent Application Laid-Open No. 2006-279381 discloses a technique whereby an access point changes CW parameters in response to an increase in terminal stations under its own control. The specification of Japanese Patent Application Laid-Open No. 2009-118351 describes a technique whereby a communication device having a QoS function and located in the vicinity receives beacon information and collects information (a QBSS load element) as to whether or not the device has the ability to accommodate control for the service quality (QoS) included in the beacon information. With the technique described in Japanese Patent Application Laid-Open No. 2009-118351, a communication channel accommodating a small number of communication devices associated with the communication channel is specified in accordance with information that has been collected for every frequency, and communication is carried out using this communication channel.
As mentioned earlier, a dual mode device can itself be an access point and can participate as a station in a network constructed by another access point. However, if the communication channel used by a network constructed by a dual mode device in which the device is itself the access point and the communication used by a network constructed by another access point are identical or interfere with each other, then the networks will interfere with each other's communication.
In such cases, wireless priority control (QoS) cannot be achieved in each of the networks. For example, as a consequence of interference with an adjacent network, a problem will arise in which data communication requiring immediacy, as in the case of control data and moving-image data in a TV game or the like, cannot be handled preferentially. Further, if communication in a network constructed by a dual mode device itself is performed at a priority higher than that of communication in an adjacent network, then communication which the adjacent network is attempting to perform preferentially will no longer be able to be performed preferentially.